The present invention relates to the vacuum decarbonization of high-chromium and high nicket-chromium stainless steels.
The technical interest in steels of this kind having carbon contents of less than 0.03% is due to their high corrosion-resistance. One difficulty in making them consists in the undesirable chromium slagging that occurs when oxygen is blown into or onto the surface of the hot metal, if the oxidation is continued down to the desired low final carbon content. FIG. 1 shows the limits to which the carbon can be oxidized at a given Cr content at normal pressure and at temperatures of 1700.degree., 1800.degree. and 1900.degree. C. FIGS. 1 to 3 are contained in "Stahl und Eisen" 88 (1968), pp. 155-156). Accordingly, for a chromium content of 18%, it is possible to decarbonize down to 0.22% C at 1700.degree. C without loss of chromium, and down to 0.13%C at 1800.degree. C and 0.08%C at 1900.degree. C.
The chromium can be protected against oxidation by decarbonizing at reduced pressure, instead of increasing the temperature, because a low pressure modified the conditions of equilibrium of the reaction EQU Cr.sub.3 O.sub.4 + 4 C = 3 Cr + 4 CO (1)
in such a manner as to favor the formation of carbon monoxide, i.e., so as to favor the oxidation of the carbon. FIG. 3 shows how this equilibrium shifts with the pressure at a temperature of 1800.degree. C. Accordingly, at a chromium content of 18%, the decarbonization can be performed down to a carbon content of 0.07% at a pressure of 0.5 atm., and down to a carbon content of 0.02% at a pressure of 0.1 atm., without the occurrence of chromium oxidation.
If a vacuum decarbonization is performed, i.e. an oxidation of carbon at reduced pressure according to the reaction EQU C + O = CO (2)
the oxygen balance of such a process must be taken into consideration, since the amount of oxygen that is required for the oxidation of the carbon has to be fed under a vacuum to the molten metal.
FIG. 2 is intended to explain this. It shows, in addition to the equilibrium curves computed for the stated pressures, concentrations and temperatures, a large number of experimental points. The concentrations of the points that lie around the equilibrium curves for a pressure of one atmosphere are adjusted in the decarbonization process at normal pressue. The contents of the points around the isobars for 0..1 atm. are achievable after vacuum decarbonizing in a large industrial degassing installation. The line a-b indicates the trend of the stoichiometric transformation of carbon and oxygen. The concentration movement under vacuum runs parallel to this line. If more carbon is to be removed in the vacuum then corresponds to this line, then additional oxygen must be put in during the vacuum treatment, over and above the oxygen that is dissolved in the molten metal after decarbonization at normal pressure, which is indicated in FIG. 3. The industrial use of a vacuum for the decarbonization of high-chromium molten metals is in the prior art and has been described in the literature. In this procedure, remelting furnaces such as vacuum-reduction furnaces are used (ef. "Die Hereaus-Vakummschmelzen; 1923-1933," published 1933, and "Giesserei" 53, (1966) pp. 229-234), and it has been proposed that high-carbon ferro-chrome be carbonized in an evacuated converter for the production of low-carbon ferro-chromium (German Pat. No. 675,565). The latter proposal has not yet been exploited industrially on account of the difficulties involved in the hermetic sealing of an entire converter.
In recent times proposals have been made for the use of large industrial steel degassing installations of the prior art (such as installations for siphon degassing, continuous-flow degassing, ladle degassing or degassing during tapping or teeming (cf. U.S. Pat. No. 3,335,132). Nothing is said, however, about the absolute necessity of supplying additional oxygen over and above the dissolved oxygen, and the difficulties arising out of this oxygen need, even though the decarbonizing capacities described are impossible of achievement without meeting this oxygen demand. In fact, it is even stated that the addition of solid oxygen carriers, such as iron ore, etc., has proven to be ineffective.
In the meantime, however, vacuum decarbonization with the addition of oxygen carriers in solid forms, such as iron ore or chromium ore, in a large industrial degassing installation, pertains to the state of the art. Relatively large quantities of oxidizing agents are used. On the assumption that the ores that are added are practically free of gangue, i.e., that they are pure Fe.sub.2 O.sub.3 or Fe.sub.3 O.sub.4, 4.5 to 5 kilograms of iron oxide have to be added for each kilogram of carbon. In the case of a 30 metric ton melt having a starting carbon content of 0.30%, approximately 90 Kg of carbon has to be oxidized out in order to achieve a final carbon content of 0.01%. The amount of oxide required for this purpose amounts to about 450kg. The amount of additional heat needed if such large quantities of ore are added is very great. The temperature losses during vacuum treatment amount to about 150.degree. C in the case of a heat weight of 30 tons. It is quite possible to produce the necessary overheating of the metal in the furnace (e.g., arc furnace of LD converter) that precedes the vacuum treating equipment, but this entails intolerable wear on the refractory lagging in these furnaces, especially in the case of series production, since final temperatures of 1900.degree. C and more must be reached before tapping. On the other hand, however, there is a technical interest in adjusting the carbon content to over 0.30% prior to the vacuum treatment, because, on the basis of FIG. 1, only when the carbon concentration amounts to more than this figure can the slagging of the chromium and hence undesired chromium losses be prevented when decarbonizing at normal pressure at low temperatures of less than 1700.degree. C and at chromium contents of about 20%.